Spray is a common gas-liquid two-phase flow phenomenon in the filed of energy environment at present, for example, atomization, evaporation and combustion of liquid fuel, gas-liquid mixture and absorption in a desulfurization and denitrification rainmaker, etc. In the art, there have been various techniques for measuring a spray field. Conventional contact measurement methods include: immersion, tracking, sedimentation, freezing, wax melting, instant sampling, etc. The methods as mentioned above may damage the original flow field, resulting in errors and great limitations in application, and are thus not applicable to the present measurement requirements. However, many laser measurement techniques have broken the above limitations and have advantages of no interference to the original flow field, high precision, real-time and quick measurement, large volume of information, availability of quantitative calculation and the like.
According to the measurement category of laser techniques, the common methods for measuring the size of spray droplets include Melvin granulometer, laser MIE scatter, laser-induced fluorescence, laser holography, laser photomicrography, Interferometric Laser Imaging Droplet Sizing (ILIDS), and Phase Doppler Anemometry (PDPA or PDA); the common methods for measuring the refractive index of spray droplets include V-shape prism, glancing incidence (Abbe refractor), and interference fringe (Newton ring); the methods for measuring the temperature of spray droplets mainly include fluorescence and rainbow scattering, the latter can measure the size of droplets or size distribution simultaneously and is classified into two forms, i.e., standard rainbow and a global rainbow; in addition, the common methods for measuring the concentration of a spray field include pseudocolor, shadow and Computerized Tomography (CT); and the common methods for measuring the speed of a spray field include Laser Doppler Velocimetry or Laser Doppler Anemometry (LDV or LDA), Light Speckle Velocimetry (LSV), Phase Doppler Anemometry (PDPA or PDA) and Particle Image Velocimetry (PIV).
The principle of the Global Rainbow Technique (GRT) is that light is irradiated onto spherical particles, a part of the light is incident on the balls and then emitted from the balls after reflected primarily by the inner surfaces of the balls (known as First-order Rainbow) while another part of the light is reflected by the outer surfaces of the balls. The reflected light and the emergent light resulted from the primary internal reflection are interfered with each other to form a series of intensity oscillating ripples. As there are multiple kinds of obvious oscillation of different frequency in the rainbow signals, it is required to filter high-frequency oscillation structures to form smooth rainbow signals. By recording the prolonged time of exposure and the expanded clear aperture, the rainbow of thousands of droplets having a certain size distribution is recorded. As the scattered light of various particles is overlapped with each other, the high-frequency ripple signals attached to a single particle of first-order rainbow are eliminated, so that the rainbow signals may be smoothened, and then the average refractive index, size distribution, average temperature and other parameters of the spray droplets are thus inverted.
As the global rainbow technique has a unique advantage of measuring both the size and the reflective index to further invert the temperature and other parameters of the droplets, in the measurement of the spray flow field, the global rainbow technique and the related applications attract the attention of researchers all over the world. Van Beeck et al., from University of Brussels, have measured the liquid-liquid suspension, two-phase jet flow, and droplet evaporation and diffusion based on the rainbow technique. G. Grehan team, from France, has done a lot of research on the global rainbow technique in measuring the reflective index, reflective index gradient, temperature and size distribution of droplets, and has applied the global rainbow technique in complicated and harsh sites for measurement. S. Bakic et al., from Darmstadt University of Technology in Germany, have applied the global rainbow technique to the component measurement during the evaporation of bi-component droplets. J. Wilms et al., from Stuttgart University in Germany, have measured the change of component during the evaporation of a single bi-component droplet based on the rainbow technique. The S. Sankar team, from America, has done research on the heating and evaporation characteristics of fuel droplets based on the rainbow temperature measurement technique, and has used the rainbow technique together with the PDA in measuring the combustion particulates. H. Lohner et al., from Bremen University in Germany, have done research on the non-spherical degree of droplets when measuring the liquid-liquid suspension based on the rainbow technique. V. Bodoc et at., from France, have researched the evaporation of bi-component droplets under a turbulent environment within a passage based on the rainbow technique. Wu Xuecheng, Wu Yingchun, et al., from Zhejiang University, have done simulation and experimental researches on the measurement of size, concentration and temperature of spray droplets, the measurement of components of complex bi-spray, and the volume ratio of all components, based on the global rainbow technique.
So far, the global rainbow technique is restricted to single-point measurement. The global rainbow technique may provide a better test tool for researches on the non-steady-state complex spray flow fields If one-dimensional, two-dimensional and even three-dimensional global rainbow techniques are further developed, which will be of great significance in the further in-depth study of the spray mechanism.